I own two XPV's (a 30 and a 40) and two Provaris (a V2 and a P3).
They are all really great devices, and do what they are supposed to do within their operating envelope. They both have a pretty loyal following, but I'd say that Protovapor has probably sold more units, although that's irrelevant to the end user (other than as a gauge of popularity).
The XPV 40, of course, adds temperature control, which a lot of people, including myself, are using, and experimenting with. I've found it useful, and the fact that some factory coils are now going to be produced with Ni200 wire to take advantage of temperature control is pretty exciting.
I'd say that with factory tanks and coils, performance wise, they are indistinguishable from one another, although the "boost" on the P3 is something that has good potential.
With rebuildables, again, I believe they are identical in performance at nominal power settings. A Kayfun with a 1.2 ohm coil on a Provari V2 at 3.8 volts vapes just like it would on a P3 at 12 watts or an XPV at 12 watts.
The question is where do you want to end up at the end of the day? If all you're looking for is a good, solid, device, and a nice, enjoyable vape, it's a tossup. If you're interested in higher power and rebuilding with Ni200 wire, then the XPV is your huckleberry.
And yes, they both do have fantastic customer service, and neither company will leave you hanging.